A synchronizing ring of such a kind has an annular structural part and a frictional layer. Some synchronizing rings have an annular structural part and a frictional layer formed on the inner circumference of the structural part, some synchronizing rings have an annular structural part and a frictional layer formed on the outer circumference of the structural part, and other synchronizing rings have an annular structural part and frictional layers formed on the outer and inner circumferences of the structural part, respectively. In any one of those synchronizing rings, the frictional layer is brought into frictional engagement with the tapered portions (conical portions) of the speed gears.
The frictional layer is provided in its working circumference to be in engagement with the tapered portion of the speed gear with a plurality of circumferential grooves of a trapezoidal cross section for cutting an oil film. Top lands are formed between the grooves in the frictional layer.
If the top lands are abraded, the area of contact between the frictional layer and the tapered portion increases and an oil film is liable to be formed. Consequently, such a synchronizing ring having abraded top lands needs much time for bringing the twospeed gears to the same rotating speed.
Therefore, in most cases, the frictional layer is formed of a highly abrasion-resistant, frictional metal easy to machine to form grooves therein, such as iron, aluminum bronze, and high-strength brass.
In most recent transmissions of a high output capacity, a high pressure is exerted on a synchronizing ring by the tapered portion of a speed gear. Therefore, the frictional layer of the synchronizing ring is abraded rapidly and hence, in some cases, the synchronizing ring is unable to withstand a long period of use. Frictional layers of high-strength brass are particularly rapidly abraded.
Some synchronizing rings have a structural part and a frictional layer formed of a material containing resin, metal or paper as a base material and bonded to the structural part.
All those synchronizing rings need to be fabricated by bonding a frictional layer to a structural part. Therefore, the manufacture of those synchronzing rings needs much manhours and high costs.
A method of manufacturing a synchronizing ring proposed to solve the foregoing problems is disclosed, for example, in Japanese Unexamined Patent Publication No. 223105/1988.
This previously proposed method of manufacturing a synchronzing ring comprises the following steps.
First, a self-holding molded ring, which serves as a frictional layer, is formed by compacting a mixture of a sintered powder and a bond or by presintering a sintered powder.
Secondly, the molded ring is pressed in a support ring (structural part) to form an assembly of the molded ring and the support ring.
Lastly, the assembly of the molded ring and the support ring is sintered to unify the molded ring and the support ring.
This method of manufacturing a synchronizing ring has the following problems.
(1) The method requires much time and labor and hence it is difficult to manufacture the synchronizing ring at a low cost because the method needs both the process for compacting a sintered powder and the process for pressing the molded ring in the support ring.
(2) The molded ring, i.e., the frictional layer, needs tube formed of metal so that the molded ring may not be broken by a pressure that is applied thereto when the molded ring is pressed in the support ring and the frictional properties and abrasion resistance of the metal must be sacrificed to strength. Consequently, the porosity of the frictional layer is reduced and the frictional layer is unable to secure a satisfactorily high frictional effect.